Be aware that even if your protagonist doesn’t kill/defeat the villain in your story, he should at least be equipped to. Depending on your genre, you may not want to depict your main character defeating the bad guy in a gruesome or fatal way. Maybe it’s not an appropriate scene for your target audience, or maybe you don’t want to have to show your character enduring the guilt to follow.
For this reason, sometimes authors defeat the villain by having another character commit the act (your main character is holding the gun on the villain, but someone else steps in and fires), by inserting a freak accident (both characters are fighting on a rooftop, and the bad guy accidentally falls to his death), by having the villain self-destruct (the villain’s plan spins out of control; the villain is killed by his own network of people; the villain chooses to “go out in a blaze of glory” due to being cornered by the protagonist), or by some other means.
So while your protagonist may not always play a “trigger-pulling” role in your villain’s defeat, your main character should demonstrate a worthy effort that leaves no room for doubt regarding the events of the climax. Readers should leave the scene feeling convinced that your protagonist didn’t just “get lucky” and that had things happened differently, your character would have still found a way to come out on top.
Writing the end of a manuscript can leave some authors feeling refreshed. ("It's almost over! Yay, me! I'll tell all my followers on Twitter!").
For most of us, there's nothing left of our nails to bite. The ending of a story means there's a lot to wrap up. And we genuinely want to do it right.
When I get to the end of my novel, I ask myself three things (okay, I have a load of questions I ask myself when I get to the end, usually concerning specific elements of the storyline, but for the sake of speaking generally, I'll shrink that list to three things):
1) "Have I left anything unresolved?" In other words, I make sure I haven't abandoned (or simply forgotten to address) important storylines. If a reader has been holding out to the end to find out the resolution of something (even if it's small, such as indiscernible markings on a wall that my character noticed and wondered about but never again revisited) and I don't provide that, then I've failed to meet reader expectations.
2) "Did I throw in last-minute conflicts (if it's a series) or did I keep from throwing in last-minute conflicts (if it's a novel)?" If your reader has picked up a novel written by you, he or she wants the climax to focus on the main tension you've been building to. Introducing new and unexpected conflicts AFTER the climax will not be handled well by the reader (if they're reading a series, however, they will expect and be looking for this unexpected bit of tension as they're nearing the end of the book).
3) "Is my storyline resolved but not perfect?" Although it's fiction, we should be conveying, to some degree, how things happen in real life. When main conflicts end, the world is brighter, but it's never perfect. People are still left with scars. Graves still have to be visited. Sometimes bitterness does not easily fade. A reader will want to see your character has come out on top, but a reader will not expect everything to be glorious. The depth of resolution will depend on your genre. If your audience has been reading an upbeat storyline, they'll expect a nearer-to-perfect ending. If your audience has been reading a thriller/suspense, they're going to be shocked by a "Kumbaya" ending.
The ending of your story doesn't have to be scary. Think about what your reader will be wanting most, and do your best to fulfill and exceed those expectations.